Following [1]}, we determine the amplification matrix by associating with \(\lambda =\theta ^2\)  the eigenvalues of the matrix \(M^{-1}K\) . The damping term's eigenvalues \(M^{-1}C\)  are \(\zeta \theta \)  where \(\zeta \)  is the damping coefficient. Without loss of the proof's generality, we set \(\zeta =0\)  and ignore damping. Then, substituting (REF ) into (REF ), we obtain a system of equations at each time step as
\(A{U}_{n+1}=B {U}_n+{F}_{n+\alpha _f},\) 
